Our good friend at Pawsitive Life wants to know why old milk curdles in coffee. First, here’s the theory:

So, what makes milk curdle at all?

Curdling is the coagulation of milk proteins. Though usually thought of as undesirable, the controlled process is necessary for the production of cheese and yogurt. The proteins responsible in cow’s milk belong to a family called casein.In fresh milk, casein carries a negative electric charge and exists in spheres called micelles centre around positively charged calcium ions.

Even after pasteurization, milk contains bacteria which slowly convert the milk sugar lactose into lactic acid. Milk’s normal pH of 6.5 drops over time, and as it nears 4.6, the casein begins to lose its negative charge. Without the attractive force maintaining the micelles, the spheres gradually break down, allowing the casein to form new bonds in long chains, i.e. the milk curdles.

Brewed coffee has a pH of around 5 and contains phenolic compounds called tannins which readily bond and cross-link proteins. Tannins give body to coffee, tea, and red wine by binding to saliva proteins (they also stain your teeth). When partly-acidified old milk is added to coffee, the combined effect of reduced pH and astringency can instantly push old milk over the edge and curdle it.